Sumatra PDF is a PDF, ePub, MOBI, CHM, XPS, DjVu, CBZ, CBR reader for Windows

PDFs look washed out when I open them

I would like to increase the darkness of the lines on the PDF. The faint lines only happen in a very few of my PDFs

Thin zero width lines will appear grey as they are anti-aliased by MuPDF settings in SumatraPDF thus will be expected to drop density potentially by upto 50% (half black half white) It is a ignorant / forgetful / careless author that allows that to happen. For CAD vectors and thus images there are standard line thicknesses to be upheld.
In some cases it is possible to process the file to restore some minimum values and often I will use cpdf thickness commands or xchange editor to correct any pdf that I need correcting. Also, it is possible to reprint the source into fixed resolution images so those lines become thicker, but in such cases that may result in losing the ability to search text.
SumatraPDF uses default values that have made some minor difference between versions, Many users have pointed out override settings in other viewers, that either alias or artificially enhance line thickness but SumatraPDF has historically respected the source file settings.
You could ask the authors (or their publishers) to respect industry guidance on graphic standards